Vitreous bonded abrasive grinding wheels, as well as other vitreous bonded abrasive articles (e.g. honing stones), have been known in the art for a long time. Such wheels and articles have long been the subject of efforts to improve both materials and methods for their manufacture to gain greater grinding performance, higher utility, greater life and improved economics. Improved abrasive grains and methods for their production, as well as improvements in the composition and properties of vitreous bond materials have resulted in greater grinding performance, lower cost, improved work products and greater wheel life in many cases. However, increases in utility and performance continue to be sought, particularly as advances in technology place ever greater demands on precision, accuracy and performance of devices and their ground component parts and increased competition places ever greater emphasis on economic advantages in wheel performance and grinding operations.
Essentially, a vitreous bonded grinding wheel and other vitreous bonded abrasive articles, have abrasive grain or grit, e.g. alumina abrasive, bonded together by a vitreous material. Other functional materials, such as for example, solid lubricants, grinding aids, extreme pressure agents and hollow fillers ("bubbles"), sometimes are included in the wheel or article. In the typical known method of making a vitreous bonded abrasive grinding wheel or article, abrasive grain, bond precursor (e.g. frit or other vitrifiable materials), temporary binder (e.g. aqueous phenolic resin binder), and, selectively, other functional materials and/or pore inducers, are blended together to form a uniform mixture. This mixture is then placed in a mold generally defining size and shape of the article and compacted into a self-supporting article held together by the temporary binder. This compact, or "green" article is dried and then placed in a kiln to be heated, i.e. fired, under a particular cycle of time, temperature and atmosphere to burn off the temporary binder and any organic pore inducer present and to vitrify the bond precursor. The heating cycle depends upon the composition of the wheel or article and may vary with the abrasive grain, the composition of the vitrifiable material, the additives used and the size and shape of the wheel.
It is known in the art to produce vitreous bonded grinding wheels of different grades tailored to meet particular grinding conditions and requirements. These grades are broadly characterized from soft to hard. Thus grinding a soft metal workpiece (e.g. copper, aluminum) often required a wheel grade different (e.g. softer) than a wheel for grinding a hard or tough metal workpiece (e.g. nickel, stainless steel). The grade of the wheel is dependent upon a number of manufacturing, chemical and physical factors including but not limited to firing conditions; the composition of the abrasive grain; grain size; grain concentration in the wheel; vitreous bond matrix composition; concentration of vitreous bond matrix in the wheel; porosity of the wheel; pore size; and adhesion between the grain and vitreous bond matrix. These different grades can exhibit different physical properties and different grinding performance. Notwithstanding variations among grades, improved grinding performance is sought for all grades of vitreous bonded abrasive grinding wheels. Particular performance improvements include, for example, increased retention of wheel forms such as are used to produce contours in finished workpieces, reduced frequency of wheel dressing to maintain desired cutting performance, improved wheel life, increased metal removal rate, increased grinding ratio and lower power consumption.
As previously noted, practitioners in the art have sought performance improvements through variations in the composition of the vitrifiable material for producing the vitreous bond matrix. Such changes affect the strength of the bond retaining the abrasive. A vitreous bond matrix that is too strong can prevent or reduce the occurrence of grain fracture, a mechanism by which new sharp cutting edges are produced during use. Reduced occurrence of grain fracture can result in reduced metal removal and workpiece burning (i.e. surface discoloration) of metallic workpieces. On the other hand a vitreous bond matrix too weak can lead to premature grain loss during grinding, resulting in increased wheel wear and consequent low grinding ratio (i.e. ratio of volume of metal removed to volume of wheel lost during a grinding period).